1. Field of the Invention
This invention relates to a material of a membrane for an X-ray mask used in a fabrication process of a fine pattern of a semiconductor device, such as a VLSI (very large scale integrated circuit). The word "membrane" is used to mean a film spreading in a space.
2. Description of the Related Art
As is well known, an X-ray proximity exposure has been employed in a precise transcription to obtain a finer pattern, such as of sub-micron resolution. Accordingly, a mask for X-ray exposure has also been developed in place of an optical mask using quartz substrate. A structure and the fabrication process of the mask for the X-ray exposure is shown in FIGS. 1 and 2. On a wafer 10, which is usually made of silicon 110 mm in diameter and 4.about.5 .mu.m thick, there is formed a layer 12 made of a material transparent to the X-ray. Next, a mask pattern 13 made of a metal which is not transparent to the X-ray is formed on the transparent layer 12. Material for the formed mask pattern 13 is chosen from heavy metals, such as gold, tantalum and tungsten, which prevent X-rays from penetrating therethrough; therefore, the mask pattern 13 acts as a mask. Next, the central portion 10' of the wafer 10 is removed typically by a reactive ion etching as shown in FIG. 2. Thus, the central portion of the layer 12 remains as a membrane, and the peripheral portion of the wafer 10 remains as a frame 11 to support the membrane. Thus, the X-ray mask is formed.
Quartz, which has been used for the substrate of an optical mask, cannot be used as an X-ray mask, since quartz absorbs X-rays. If beryllium or boron, both of which are transparent to X-rays, is used for the membrane, an optical alignment of the mask cannot be carried out because these materials are not optically transparent. Therefore, materials suitable for the membrane of the X-ray mask have been intensively investigated, as follows:
1. A polymer of BN (boron nitride), such as reported by Morita et al. in The 33rd Joint Convention of Institutions of the Applied Physics, Spring, 1961, Japan.
2. An amorphous BN:H (hydrogenated boron nitride) or BNC:H (hydrogenated boron nitride carbide), where the notation ":H" indicates that the H (hydrogen atom) is made from a hydride source gas in a CVD (Chemical Vapor Deposition) processing and terminates a dangling bond. This material is reported, for example, ibid or by K. Montesser et al. in "Thin Solid Films" 117(1984) 311-317.
However, there have been discussed only the transparency characteristics to X-rays or visible light. On the other hand, the X-ray membrane must be transparent and, at the same time, have the following mechanical characteristics (which have not heretofore been fully satisfied):
a. Stiffness (represented by Young's modulus and high elastic limit (represented by tensile strength and rigidity).
b. Proper residual tension at the operating condition.
The above-mentioned stiffness and residual tension are required by the following reason. As shown in FIG. 3, when the X-ray mask is in use having X-rays 14 irradiating from the back of the mask (which is upside-down of FIG. 2), the clearance between the membrane 12 having the mask pattern 13 thereon and the resist film 22 on a wafer 21 must be kept as narrow as approximately 10 .mu.m in order to attain a sharp resolution. If an adequate amount of stiffness and tension is not maintained in the membrane, the membrane may droop downwardly towards the resist film 22, causing a non-uniform clearance thereof, and consequently the clearance must be widened to avoid the pattern touching the resist. This results from the fact that the membrane may be as thin as 4 to 5 .mu.m and have a relatively large diameter of tyically about 60 mm. The resulting residual tension is produced by the intrinsic internal stress and larger thermal shrinkage of the membrane material than that of the frame 11. This is because the membrane is formed at a high temperature, such as 400.degree. C. If the residual tension is negative, i.e. compressive, the membrane is likely to have wrinkles. Therefore, a need exists for a material having a large stiffness as well as a large residual tension therein. The preferable amount of the stiffness of the membrane is approximately 3.times.10.sup.12 dyne/cm.sup.2 of Young's modulus. The preferable amount of the residual tension is approximately 0.5 to 1.0.times.10.sup.9 dyne/cm.sup.2, which must be lower than the tensile strength of the membrane material so that a neutral break of the membrane can be avoided.
The above-mentioned transparency to visible light is required because the X-ray mask is aligned with the wafer by projecting visible light through the membrane onto the resist film 22. Therefore, unless the membrane is adequately transparent, the required resolution necessary for alignment cannot be achieved.
One of the problems with the known amorphous BN:H materials is that it is not adequately transparent to light used for the alignment, while keeping a proper amount of stiffness and residual tension therein. In other words, if the amorphous BN:H is made thick for gaining strength, transparency is degraded. Among the above-described requirements, stiffness is the most serious. Therefore, a need exists for improvements in stiffness.